TS-2788: proxy.config.alarm.bin should be reloadable

Stop caching the value of proxy.config.alarm.bin and friends, so
it is reloadable.

Add a helper API, RecConfigReadBinDir(), to consistently handle
proxy.config.bin_path. Update all the places that uses
proxy.config.bin_path to use the new API.

Update the alarms configuration documentation. Remove various
unnecessary buffers and string caches.